Modified Stirling cycle refrigerators, such as those of the type described in U.S. Pat. No. 3,074,244 have proved to be the most reliable, most efficient and lowest cost cryogenic refrigerators available. They have the additional important advantages of operating using non-hazardous working gases such as helium or nitrogen and of not requiring any condenser or evaporator coils. Cryogenic refrigerators of the type described in the above patent regularly achieve temperatures below 100.degree. K. at the cold head and have been operated at temperatures below 30.degree. K. at the cold head.
Such refrigerators, depending on capacity, include one or more sets of compressor and expander pistons. Each expander piston is generally protected from the extreme cold temperatures of the working gas in the vicinity of the cold head by an extender portion, sometimes referred to as the expander dome, mounted on the face of the expander piston toward the cold head. The extender is normally made of a thermally insulating material such as wood or plastic which is able to withstand extremely cold temperatures and has a cross-sectional diameter slightly smaller than that of the expander piston. The length of the extender is chosen to be long enough to provide adequate thermal isolation to the expander piston and so that the volume of the expander cylinder is appropriate for operation of the system.
Although the cryogenic refrigerators in accordance with the invention operate very well in most applications, in larger capacity refrigerators the reciprocating mass of the extender becomes large, which tends to reduce the efficiency and reliability of the unit. In addition, if the extender is at all porous it may absorb some of the refrigerant which reduces the compression of the refrigerant and may adversely affect the properties of the extender.